1. Field of the Invention
The present invention relates in general to a vitrified grindstone which has a vitrified abrasive structure impregnated with a cured resin, and a method of manufacturing such a vitrified grindstone.
2. Discussion of the Related Art
There is known a vitrified grindstone having a vitrified abrasive structure, in which abrasive grains and an optionally used aggregate are held together by a glassy inorganic vitrified bonding agent, and in which a multiplicity of pores or voids are formed between the abrasive grains. Owing to excellent properties of the vitrified grindstone, such as a relatively high degree of strength with which the bonding agent holds the abrasive grains together, and a relatively easy dressing operation, the vitrified grindstone is widely used for precision grinding operations. In recent years, there is a need for a vitrified grindstone capable of performing a grinding operation with improved efficiency, to meet a demand for shortening of the required grinding time.
On the other hand, there has been proposed for practical use a vitrified grindstone wherein the proportion or content of the inorganic bonding agent is made relatively high for increasing a grade of the vitrified grindstone and also a degree of strength with which the inorganic bonding agent holds the abrasive grains together. This vitrified grindstone has a relative high hardness, and can meet, to some extent, the above-indicated need for improving the grinding efficiency. However, this vitrified grindstone is not completely satisfactory. Namely, the use of the inorganic bonding agent in a relatively large proportion assures an increase in the strength of the vitrified grindstone, which contributes to an improvement in the grinding efficiency, but considerably reduces the porosity of the vitrified abrasive structure and results in difficult or insufficient fracturing and removal of the abrasive grains, leading to relatively easy glazing or clogging of the surface of the vitrified grindstone, relatively easy chipping of the abrasive structure, relatively difficult dressing operation of the vitrified grindstone, and other drawbacks encountered during use of the vitrified grindstone as a grinding tool. In addition, the use of the inorganic bonding agent in the relatively large proportion is likely to cause various drawbacks during the manufacture of the vitrified grindstone, such as cracking or deformation of the grindstone and insufficient removal or burn-out of the primary binder of the inorganic bonding agent, in the firing process. The insufficient removal of the binder may cause the manufactured vitrified grindstone to have some amount of residual carbon.
Such a vitrified grindstone with reduced porosity may be manufactured by hot-pressing of the materials of the grindstone, and may be used for performing a highly efficient grinding operation. However, this manner of manufacturing the vitrified grindstone requires special manufacturing equipment which usually suffers from low manufacturing efficiency, leading to a relatively high cost of manufacture of the vitrified grindstone, and a considerable limitation in the range of size of the vitrified grindstone that can be manufactured.
It is therefore a first object of the present invention to provide a vitrified grindstone which has a vitrified abrasive structure impregnated with a resin, and which is less likely to suffer from the above-indicated drawbacks relating to its use and manufacture and assures a high degree of grinding efficiency.
It is a second object of the present invention to provide a method of manufacturing such a vitrified grindstone.
The first object may be achieved according to a first aspect of this invention, which provides a vitrified grindstone having a vitrified abrasive structure which has pores and which includes abrasive grains and an inorganic bonding agent that holds the abrasive grains together, wherein 10-95% of a total volume of the pores is filled with a cured resin.
In the vitrified abrasive structure of the present vitrified grindstone, a suitable volumetric percentage of the total volume of the pores or voids is filled with the cured resin, so as to prevent filling of the pores with metal particles which are removed from the workpiece during a grinding operation using the present vitrified grindstone and which would otherwise be fused in the pores, causing clogging or glazing on the grinding surface of the vitrified grindstone. It is also noted that since the resin filling the pores is softer than the abrasive grains, the surface of the vitrified grindstone is comparatively recessed at local spots corresponding to the resin-filled pores, during the grinding operation on the workpiece, so that the abrasive grains adjacent to the surface of the abrasive structure gradually fracture or break down and are removed, making it possible to prevent an excessive rise of the temperature on the workpiece surface due to an excessive amount of heat of friction which would be generated between the workpiece surface and the abrasive grains that remain dull, and also prevent chipping of the vitrified grindstone while permitting easy dressing of the vitrified grindstone. The abrasive grains which are only loosely held together by the inorganic bonding agent can be tightly held together with an additional bonding force provided by the cured resin, so that the cured resin functions to avoid early removal of those abrasive grains, assuring a high grinding ratio.
It is further appreciated that the arrangement in which 10-95% of the total volume of the pores is filled with the cured resin is effective to prevent the hardness of the grindstone from being excessively hardened by the impregnation of the vitrified abrasive structure with the cured resin, while assuring a high grinding ratio. Since an excessive increase in the hardness of the vitrified grindstone is prevented, the present vitrified grindstone can be easily dressed as needed, making it possible to prevent a reduction in the service life of the vitrified grindstone. If the volumetric percentage of the pores that is filled with the cured resin is smaller than 10%, the effect of the impregnation of the abrasive structure with the cured resin cannot be expected. If the volumetric percentage of the pores that is filled with the cured resin is larger than 95%, the vitrified grindstone is excessively hardened, making it difficult to dress the vitrified grindstone.
It is to be understood that the vitrified abrasive structure of the vitrified grindstone may further include an aggregate such that the abrasive grains and the aggregate are held together by the inorganic bonding agent.
According to a first preferred form of the first aspect of the invention, 40-90% of the total volume of the pores is filled with the cured resin. In the vitrified grindstone of this first preferred form, an excessive increase in the hardness of the vitrified grindstone is more reliably prevented owing to the arrangement in which at least 10% of the total volume of the pores remains unfilled with the cured resin. Further, since at least 40% of the total volume of the pores is filled with the cured resin, it is possible to more reliably prevent the conventionally encountered drawbacks such as easy filling of the pores with metal particles, easy glazing or clogging of the surface of the vitrified grindstone, easy chipping of the abrasive structure, and easy removable of the abrasive grains. Thus, the present vitrified grindstone can be easily dressed without having to increase a load applied to the vitrified grindstone, leading to a further increased grinding ratio and providing a further improved surface smoothness of the workpiece.
According to a second preferred form of the first aspect of the invention, the cured resin consists of at least one thermosetting synthetic resin which is selected from a phenol resin and an epoxy resin, so that the vitrified grindstone has a higher degree of hardness than where the resin consists of a thermoplastic resin.
According to a third preferred form of the first aspect of the invention, the abrasive grains includes super abrasive grains (considerably fine abrasive grains) consisting of diamond abrasive grains, CBN abrasive grains, or mixture of diamond and CBN abrasive grains. The super abrasive grains preferably have Knoop hardness of at least 3000. It is preferable that the super abrasive grains have an average particle size of 20-220 xcexcm. The particle sizes of 20 xcexcm and 220 xcexcm correspond to 800 and 60 meshes, respectively. Preferably, the super abrasive grains in the vitrified abrasive structure has a concentration of larger than 10 and smaller than 230, more preferably, a concentration ranging from 20 to 200.
According to a fourth preferred form of the first aspect of the invention, the vitrified abrasive structure has a porosity of 20-75% by volume, more preferably, 30-65% by volume, before the vitrified abrasive structure is impregnated with the resin.
According to a fifth preferred form of the first aspect of the invention, the inorganic bonding agent consists of a borosilicate glass or a crystallized glass which is suitable for holding super abrasive grains together. The crystallized glass may be, for example, a glass in which willemite precipitates. The inorganic bonding agent preferably has a thermal expansion coefficient ranging from xcex1xe2x88x92(2xc3x9710xe2x88x926) to xcex1+(2xc3x9710xe2x88x926) [1/K] (where xcex1 represents a thermal expansion coefficient of the super abrasive grains), so that the super abrasive grains can be tightly bonded together by the inorganic bonding agent.
According to a sixth preferred form of the first aspect of the invention, the vitrified abrasive structure includes 15-35% by volume of the inorganic bonding agent, so that the porosity of the vitrified abrasive structure is held in the volumetric range as described above, without deteriorating the holding strength with which the bonding agent holds the abrasive grains together. The vitrified abrasive structure may include, as an aggregate, a pore forming agent such as an inorganic balooning agent or other inorganic hollow substance.
The second object may be achieved according to a second aspect of this invention, which provides a method of manufacturing a vitrified grindstone having a vitrified abrasive structure which includes abrasive grains and an inorganic bonding agent that holds the abrasive grains together. This method comprises (a) a step of preparing a mixture of a liquid resin and a liquid diluent which is other than the liquid resin and which dilutes the liquid resin, such that the mixture includes 10-95% by volume of the liquid resin; (b) a step of impregnating the vitrified abrasive structure with the mixture; and (c) a step of curing the liquid resin contained in the mixture with which the vitrified abrasive structure is impregnated.
According to this method, the vitrified abrasive structure is impregnated with the mixture of the resin and the liquid diluent in the impregnating step, and the resin contained in the mixture is then cured or hardened in the curing step, for thereby making it possible to obtain the vitrified grindstone having the vitrified abrasive structure whose pores are partially filled with the cured resin, wherein a volume ratio of the cured resin to the pores is determined depending upon a kind of the liquid diluent and a mixing ratio of the liquid resin to the liquid diluent. Since the mixture includes 10-95% by volume of the liquid resin, 10-95% of a total volume of the pores is filled with a cured resin, as a result of the impregnation of the vitrified abrasive structure with the mixture. Thus, a suitable percentage of the total volume of the pores or voids of the vitrified abrasive structure is filled with the cured resin, so as to prevent filling of the pores with metal particles which are removed from the workpiece during a grinding operation using the present vitrified grindstone and which would otherwise be fused in the pores, causing clogging or glazing on the grinding surface of the vitrified grindstone.
It is also noted that since the resin filling the pores is softer than the abrasive grains, the surface of the vitrified grindstone is comparatively recessed at local spots corresponding to the resin-filled pores, during the grinding operation on the workpiece, so that the abrasive grains adjacent to the surface of the abrasive structure gradually fracture or break down and are removed, making it possible to prevent an excessive rise of the temperature on the workpiece surface due to an excessive amount of heat of friction which would be generated between the workpiece surface and the abrasive grains that remain dull, and also prevent chipping of the vitrified grindstone while permitting easy dressing of the vitrified grindstone. The abrasive grains which are only loosely held together by the inorganic bonding agent can be tightly held together with an additional bonding force provided by the cured resin, so that the cured resin functions to avoid early removal of those abrasive grains, assuring a high grinding ratio.
It is further appreciated that the present method merely requires, in addition to the steps performed in the conventional method of manufacturing the vitrified grindstone, the impregnating step and the curing step, and does not require conventionally used special equipment such as hot pressing equipment, which usually suffers from low manufacturing efficiency and a limitation in the range of size of the vitrified grindstone that can be manufactured. Still further, the volume ratio of the cured resin to the pores, which ratio is held in a range of 10-95%, is effective to prevent the hardness of the grindstone from being excessively hardened due to the impregnation of the vitrified abrasive structure with the cured resin, while assuring a high grinding ratio. Since an excessive increase in the hardness of the vitrified grindstone is prevented, the vitrified grindstone can be easily dressed as needed, making it possible to prevent a reduction in the service life of the vitrified grindstone. If the above-described volume ratio is smaller than 10%, the effect of the impregnation of the abrasive structure with the cured resin cannot be expected. If the volume ratio is larger than 95%, the vitrified grindstone is excessively hardened, making it difficult to dress the vitrified grindstone.
Since the hardness of the vitrified grindstone manufactured according to this method is determined depending upon the volumetric percentage of the pores filled with the cured resin, it is possible to manufacture vitrified grindstones having respective grades different from each other, by impregnating vitrified abrasive structures identical in grade with each other, with respective mixtures having different mixing ratios of the liquid resin to the liquid diluent. Further, since the pores of the vitrified abrasive structures are not fully filled with the resin, the hardness of the vitrified grindstone can be changed even after the manufacture of the grindstone has been completed, for example, by additionally impregnating the grindstone with a suitable amount of the liquid resin. It is accordingly unnecessary to manufacture, as end products, vitrified grindstones having different grades for respective different applications, and it is possible to suitably change the grade of each of vitrified grindstones which have been prepared in the same manner to have the same grade.
Preferably, the mixture prepared in the mixture preparing step includes 40-90% by volume of the liquid resin, so that 40-90% of the total volume of the pores is filled with the cured resin. Since at least 10% of the total volume of the pores remains unfilled with the cured resin, an excessive increase in the hardness of the vitrified grindstone is more reliably prevented. Since at least 40% of the total volume of the pores is filled with the cured resin, it is possible to more reliably prevent the conventionally encountered drawbacks such as easy filling of the pores with metal particles, easy glazing or clogging of the surface of the vitrified grindstone, easy chipping of the abrasive structure, and easy removable of the abrasive grains. Thus, the vitrified grindstone can be easily dressed without having to increase a load applied to the vitrified grindstone, leading to a further increased grinding ratio and providing a further improved surface smoothness of the workpiece.
Preferably, the liquid diluent is a volatile liquid, so that the liquid diluent contained in the mixture is rapidly volatilized before the curing of the resin in the curing step. The vitrified abrasive structure of the vitrified grindstone is effectively reinforced by the impregnation of the vitrified abrasive structure with the resin, since the resin is cured more efficiently than where the liquid diluent is volatized during the curing of the resin or after the curing of the resin.
More preferably, the liquid diluent is volatilized at a temperature of 40-100xc2x0 C., so that the liquid diluent in the mixture is not substantially volatilized at a normal temperature, and is rapidly volatilized at a relatively low temperature in the curing step before the resin is cured. Since the liquid diluent is not substantially volatilized at a normal temperature, each step of the method is easily implemented.